EP1625774B1 - Temperature control for an inductively heated heating element - Google Patents

Abstract

A temperature control and method of operating the temperature control, for an inductively heated heating element. The heating element is heated by an inductor to which electrical power is supplied via a control circuit, which can also be a control circuit for an induction hob or oven. The temperature control is activated at a first point in time subject to at least one electrical value of the control circuit, which depends on the temperature of the heating element. A reference value is determined at the first point in time and a comparison value and a deviation value from the reference value is determined at least one later point in time. Depending upon the deviation value, the inductor is supplied with power so that the heating element is adjusted to a substantially constant value corresponding to the reference value.

Description

The present invention relates to a method for controlling the temperature of a heating element, which is inductively heated by an inductor, which is supplied via a control circuit electrical power and a corresponding control circuit, and an induction cooktop and an induction furnace with such a control circuit.

The heating of a heating element by induction is known. In this case, a power loss of a high-frequency alternating field, which is generated by an induction coil, the so-called inductor, by magnetic coupling in a part of the heating element, leads to the heating of the heating element. This principle is e.g. used in induction hobs, in which the heat of a cooking vessel is produced in the bottom by induction.

Out US 3,781,506 is a method for measuring and controlling the temperature of an inductively heated cooking vessel in an induction cooker known. In this method, a parameter of a circuit is measured, which supplies the inductor with electrical power. This parameter is influenced by the heating of the cooking vessel, so that its value varies with a temperature change of the cooking vessel. The temperature of the cooking vessel can be determined from the measured value of the parameter based on a temperature characteristic of the parameter.

The disadvantage of in US 3,781,506 proposed method is that it works only for a cooking vessel, for which the temperature characteristic of the parameter known and for which the method has been calibrated. That is, for cooking vessels that differ in their heating behavior of the underlying characteristic of the process, the process is very inaccurate. This also applies to cooking vessels whose heating behavior changes over time due to wear.

Out US Pat. No. 6,163,019 is a resonant frequency induction furnace, in particular for melting metals, known in which, in order to achieve a preset power level of the heating elements, an initial conduction time (pulse width) of the heating elements is predetermined and then the heating elements driven with the following enlarged pulse width until the predetermined heating power is reached.

The invention has for its object to provide a method for controlling the temperature of an inductively heated heating element available, which works regardless of the state of the heating element and for different heating elements.

This object is achieved by a method of the type mentioned in that the temperature control is activated at a first time that, depending on at least one electrical variable of the control circuit, which depends on the temperature of the heating element, at this first time a reference value or a Specified value is determined that depending on the electrical quantity at least a later time a comparison value or an actual value and a deviation of this comparison value is determined by the reference value, and that the inductor depending on the deviation power is supplied, so that the temperature of the heating element is controlled to a constant value corresponding to the reference value.

Furthermore, the object is achieved by a control circuit of the type mentioned above in that the control circuit comprises a control element for activating the temperature control that the control circuit at least one measuring device for determining at least one electrical variable of the control circuit, which depends on the temperature of the heating element, in that the control circuit is designed to determine a reference value dependent on the electrical variable at an activation time of the temperature control and for determining a comparison value dependent on the electrical variable at least at a later point in time, that the control circuit comprises a comparison unit for determining a deviation of the comparison value from the reference value Reference value comprises, and that the control circuit comprises a control unit for controlling the power controller depending on the deviation, for controlling the temperature of the heating element to a reference value corresponding constant value.

The fact that, at the time of activation of the temperature control, the reference value is determined as a function of the electrical variable of the control circuit and compared with the comparison value determined at least at a later time depending on the electrical size of the control circuit, is ensured in a simple manner, that the temperature control is independent of the choice of the heating element to a temperature corresponding to the reference value. It is also advantageous that the temperature of the heating element can thus be controlled without knowing a specific temperature characteristic of the electrical variable for the heating element. In this way, the temperature control is functional even if the heating element is positioned inaccurate to the inductor.

According to a preferred embodiment it is provided that the temperature control can be activated by a user by actuation of a control element, which is in particular at least one switch or at least one touch sensor. This allows the user to determine the desired temperature of the heating element by e.g. at an induction cooking zone of an induction hob, the temperature control is activated when water begins to boil in a cooking vessel on this induction cooking zone or a food in the cooking vessel is to be kept at a subjectively determined by the user temperature. The temperature of the heating element, e.g. of the cooking vessel, is maintained after activation of the temperature control, without having to determine the absolute temperature of the heating element with a sensor. The electrical power is automatically controlled to maintain the temperature of the heating element at the reference temperature and manual readjustment of the electrical power by the user is not necessary even if e.g. During a cooking process still cold food is supplied to the cooking vessel.

Advantageously, the comparison value of the electrical variable can be determined in predetermined, in particular periodic time intervals. In this way, the accuracy of the temperature control is increased since changes in the temperature of the heating element are caused by e.g. external influences are detected at regular intervals and the inductor supplied electrical power is readjusted accordingly to keep the temperature constant.

In order to keep the expense for the temperature control low, in a preferred embodiment, the electrical quantity from which the reference value and / or the comparison value is determined, in particular is calculated, the electrical power and / or an average voltage and / or an average current These electrical variables of the control circuit can be detected particularly easily.

According to a preferred embodiment, the reference value and / or the comparison value are determined at a predetermined frequency of the electrical variable. This procedure has the advantage that frequency-dependent influences of the heating element or the determination of the reference value or the comparison value can be avoided, whereby the accuracy of the temperature control can be increased.

The invention and its developments are explained in more detail below with reference to drawings:

Fig.1

eine schematische Darstellung eines Induktionskochfeldes mit einer Steuerschaltung zur Temperaturregelung,

Fig.2

eine Systemskizze der Steuerschaltung,

Fig.3a

eine Detailskizze der Steuerschaltung,

Fig.3b

ein schematischer zeitlicher Verlauf einer Eingangsspannung der Steuerschaltung,

Fig.3c

ein schematischer zeitlicher Verlauf einer Ausgangsspannung und eines Ausgangsstromes der Steuerschaltung,

Fig.4

ein Ablaufdiagramm der Temperaturregelung des Heizelementes,

Fig.5

schematisch einen zeitlichen Verlauf der Temperaturregelung,

Fig.6

eine schematische Darstellung eines Induktionsofens mit Temperaturregelung,

Show it

Fig.1

a schematic representation of an induction cooktop with a control circuit for temperature control,

Fig.2

a system sketch of the control circuit,

3a

a detailed sketch of the control circuit,

3b

a schematic time profile of an input voltage of the control circuit,

3 c

a schematic time profile of an output voltage and an output current of the control circuit,

Figure 4

a flow chart of the temperature control of the heating element,

Figure 5

schematically a time course of the temperature control,

Figure 6

a schematic representation of an induction furnace with temperature control,

FIG. 1 shows an induction hob 1 with a control circuit 2 for controlling the temperature of a cooking vessel 3. The induction hob 1 has a glass ceramic plate 4 with four induction cooking zones 5, at the position of which an inductor 6 is located below the glass-ceramic plate. The cooking vessel 3 is heated by one of the inductors 6. To operate the inductors 6 is on a front 7 of the Glass ceramic plate arranged an operating unit 8. This operating unit 8 comprises operating elements 9 for activating and deactivating the temperature control.

As shown in Figure 2, the control circuit 2 comprises the inductor 6 for inductive heating of a heating element 3, such as the cooking vessel 3 in Figure 1, a power regulator 10 for controlling an inductor 6 supplied electrical power P, a measuring device 11 for measuring electrical quantities ν _o , i _o , P, I the control circuit 2, a control element 9 for activating and deactivating the temperature control and a control unit 12, such as a microprocessor, for controlling the power controller 10. The control circuit 2 is from a voltage source 13 with an input voltage v _i , which is an AC voltage. The power regulator 10 usually comprises a converter (not shown) which converts the input voltage v _i with an input frequency of, for example, 50 Hz into an output voltage v _o which is in a higher frequency range, eg above 25 kHz. To control the power, which is preset, for example via a rotary selector of the control unit 8, various principles are known, for example a periodic switching on and off of the output voltage ν _o , a frequency adjustment of the output voltage ν _o or a control current change. The temperature control is activated by the control element 9 by a control signal S _T to the control unit 12. The electrical variables ν _o , i _o , P, I detected by the measuring device 11 of the control circuit 2 are supplied to the control unit 12 and processed there to form a control signal for the power control S _P. Due to the control signal for the power control S _P , which is supplied to the power regulator 10, the electric power P supplied to the inductor 6 is regulated and thus a heat output W generated in the heating element 3.

FIG. 3 a shows a detailed sketch of the control circuit 2. The control circuit 2 is supplied via the voltage source 13 with the input voltage v _i . The height of this input voltage ν _i is reduced by means of a voltage divider 14, which comprises two resistors R 1, R 2, and rectified by means of a rectifier 15 Input voltage ν _r reshaped. The positions of voltage maxima V _m in a temporal course of the rectified input voltage ν _r are detected by a peak detector 16 and followed by a high-voltage insulation 17, a value of the voltage maxima V _{m is} detected. FIG. 3 b shows the profile of the input voltage v _i and the course of the rectified input voltage v _r over a time axis t. In the course of the rectified input voltage ν _r , the value of the voltage maxima V _{m is} indicated.

The electrical power P supplied to the inductor 6 is regulated by the power regulator 10 with the aid of two high-frequency switches S 1, S 2, which may be, for example, power semiconductor components. The inductor is an output voltage ν _o and it flows an output current i _o . These two electrical quantities ν _o , i _o are influenced by a change in resistance of the heating element 3, which depends on the heating elements 3 and its temperature T. The output current i _o is detected by means of a current-voltage converter 18, at the resistor R 3, a voltage vi is applied, which is proportional to the output current i _o . is. FIG. 3c schematically shows the detected time profile of the output voltage v _o and of the output current i _o . A further, alternative measured variable, which depends on the temperature T of the heating element 3, is, for example, a phase shift Δ t between output voltage ν _o and output current i _o , for example based on a zero crossing N 1 of the output voltage ν _o and a zero crossing N2 of the output current i _o can be determined. Other electrical quantities of the control circuit 2 may also be measured, which depend on the temperature T of the heating element 3, such as an average electrical power P, an average rectified current I , a maximum current I _max or a frequency of the output voltage v _o or Output current i _o .

wobei τ einen Mittelungszeitraum angibt. Der gemittelter gleichgerichteter Strom I wird gemäß $$I=\frac{1}{\mathit{\tau }}\underset{0}{\overset{\mathit{\tau }}{\int }}abs\left(i_o\right)\cdot dt,$$

bestimmt, wobei abs(i_o) einen Absolutbetrag des Ausgangsstromes i_o bezeichnet. Eine Alternative ist die Bestimmung der Wurzel des quadratischen Mittelwertes I _rms des Ausgangsstroms i_o . Die gemittelte elektrische Leistung P und der gemittelte gleichgerichtete Strom I werden durch die Messeinrichtung 11 erfasst und der Steuereinheit 12 zugeführt. In der Steuereinheit 12 wird aus der gemittelten elektrischen Leistung P und dem gemittelten gleichgerichteten Strom I ein Wert einer Funktion F wie folgt berechnet $$F=k_p\cdot \frac{P}{V_{rms}^2}+k_I\cdot \frac{I}{V_{rms}},$$

wobei k_p und k_I Konstanten sind, die experimentell bestimmt werden, um eine maximale Variation des Funktionswertes F mit der Temperatur T des Heizelementes 3 zu erzielen. V_rms bezeichnet die Wurzel des quadratischen Mittelwertes der Eingangsspannung ν _i . Es sind auch andere Funktionen F möglich, beispielsweise kann die Funktion F auch eine Impedanz von dem Heizelement 3 und dem Induktor 6 sein, die aus einem Verhältnis von gemitteiter Leistung P zu einem Quadrat des gemittelten Stromes I bestimmt wird.From the product of output voltage v _o and output current i _o , the average electric power P can be determined $$P=\frac{1}{\mathit{\tau }}\cdot \underset{0}{\overset{\mathit{\tau }}{\int }}{v}_{\mathit{O}}\cdot i_O\cdot dt.$$

where τ indicates an averaging period. The averaged rectified current I is calculated according to $$I=\frac{1}{\mathit{\tau }}\underset{0}{\overset{\mathit{\tau }}{\int }}abs\left(i_O\right)\cdot dt.$$

determined, where abs (i _o ) denotes an absolute value of the output current i _o . An alternative is the determination of the root of the root mean square I _{rms of} the output current i _o . The average electric power P and the average rectified current I are detected by the measuring device 11 and supplied to the control unit 12. In the control unit 12, a value of a function F is calculated from the average electric power P and the average rectified current I as follows $$F=k_p\cdot \frac{P}{V_{rms}^2}+k_I\cdot \frac{I}{V_{rms}}.$$

where k _p and k _{I are} constants that are determined experimentally to obtain a maximum variation of the function value F with the temperature T of the heating element 3. V _rms denotes the root of the root mean square of the input voltage v _i . Other functions F are also possible, for example, the function F may also be an impedance of the heating element 3 and the inductor 6, which is determined from a ratio of the average power P to a square of the average current I.

FIG. 4 shows a flow chart of the temperature control of the heating element 3. In a first method step TA, the temperature control is activated by a control signal S _T. This is a normal power control, leave the selected via the control unit 8 power P and passed over for power control by means of temperature control. For this purpose, in a second method step RW, almost simultaneously to the activation of the temperature control, a reference value F _{R is determined} from the current value of the function F , which is dependent on at least one of the electrical quantities ν _o , i _o , P, I of the control circuit 2, that of the temperature T of the heating element. 3 depends. In a next method step VW, a comparison value F _v from the function F and a deviation of this comparison value F _v from the reference value F _{R are} determined as a function of the electrical variable v _o , i _o , P, I. In the next method step TR, the inductor 6 is supplied with electric power P as a function of the deviation, so that the temperature T of the heating element 3 is regulated to a constant value corresponding to the reference value F _R. In a next method step DA, it is checked whether there is a signal S _T for deactivating the temperature control. If this is not the case N, the method step VW is continued. If a signal S _T for deactivating the temperature control before Y, the temperature control is terminated in the next step TE and a power control L of the electric power P is performed without temperature control with the power controller 10 according to the power P selected by the operating unit 8.

FIG. 5 schematically shows a time profile of the temperature control. At a time t 0, the inductor 6 is activated with the heating element 3 and the inductor 6 thus supplied via the control unit 8 selected electrical power P 1, which is controlled by the power controller 10 and the heating element 3 heats up to a temperature T 1. At a time t 1, the temperature control is activated by a user by operating the operating element 9, which is, for example, a switch or a touch sensor. At this first time t 1, the reference value F _R and at later times t 2 to t 7, which are advantageously at periodic intervals, in each case the comparison value F _{V is} determined. During the averaging period τ, which the measuring device 11 requires to measure M the electrical quantities ν _o , i _o , P, I , the frequency of the output voltage ν _o or the output current i _o is regulated to a predetermined value and the power control L of the power controller 10 is interrupted as long. Since the averaging period τ is typically of the order of 10 to 800 milliseconds, this period is negligibly small compared to the typical duration d of the power control L of 5 to 15 seconds.

Once the temperature control is activated, the electric power supplied to the inductor 6 is reduced from the power value P 1 to a lower power value P 2 to keep the temperature value T 1 of the heating element 3 constant. At a time t 4, the heating element 3 is cooled by an external influence, for example by supplying cold liquid to a cooking vessel 3. This cooling of the heating element 3 to a temperature value T 2 is detected by the deviation of the comparison value F _V from the reference value F _R. Thereafter, the temperature control causes an increase in the electrical power supplied to the inductor 6 to a value P 3 in order to heat the heating element 3 back to the temperature T 1. Until the temperature T 1 is reached again, the electrical power P supplied to the inductor 6 can be reduced stepwise up to a value P 4. This power value P 4 is now supplied to the inductor 6 in order to keep the heating element 3 at the constant temperature value T 1. The temperature control remains active until it is deactivated, for example by pressing the control element 9 by the user. Another possibility of deactivating the temperature control is, for example, the removal of the heating element 3 from the inductor 6, a deactivation of the inductor 6 by the user or another power specification for the inductor 6 via the operating unit 8.

As a further example of application for the temperature control of the inductively heated heating element 3, an induction furnace 19 is shown schematically in FIG. The operating unit 8 of the induction furnace 19, which is located on a front side 20 of the induction furnace, comprises the operating element 9 for activating and deactivating the temperature control. A charging port 21 of the induction furnace 19 is bounded by side walls 22, a ceiling wall 23 and a bottom wall 24, and a rear wall 26 and a door (not shown in FIG. 6). The inductors 6 are located, for example, on the ceiling wall 23 and on the bottom wall 24 of the induction furnace 19 and are covered by the heating elements 3. The inductors 6 and the heating elements 3 may also be attached to the side walls 22. Alternatively, the heating element 3 may also be a food support, such as a baking sheet, or one of the side walls 22, the top wall 23 or the bottom wall 24th

LIST OF REFERENCE NUMBERS

1

Induction hob

2

control circuit

3

Heating element, cooking vessel, food support

4

Glass ceramic plate

5

Induction cooking zones

6

inductor

7

Front of the glass ceramic plate

8th

operating unit

9

Control element for activating / deactivating the temperature control

10

power controller

11

measuring device

12

Control unit, microprocessor

13

power supply

14

voltage divider

15

rectifier

16

peak detector

17

High voltage insulation

18

Current-voltage converter

19

induction furnace

20

Front of the induction furnace

21

Charging opening of the induction furnace

22

Side wall of the induction furnace

23

Ceiling wall of the induction furnace

24

Bottom wall of the induction furnace

25

Rear wall of the induction furnace

d

Duration of performance control

F _R

reference value

F _V

comparison value

i _o

Output current of the control circuit

I

average current

I _Max

Maximum value of the current

L

Power control with the power controller

M

Measurement of electrical quantities

N 1

Zero crossing of the output voltage

N 2

Zero crossing of the output current

P

electrical power

R 1

Resistance of the voltage divider

R 2

Resistance of the voltage divider

R 3

Resistance of the current-voltage converter

S _T

Control signal for activating / deactivating the temperature control

S _P

Control signal for power control

S1

RF switch

S2

RF switch

t

timeline

Δ t

Phase shift between output voltage and output current

τ

Averaging period for temperature control

T

Temperature of the heating element

v _i

Input voltage of the control circuit

v _r

rectified input voltage

v _o

Output voltage of the control circuit

vi

Voltage proportional to the output current

V _m

Maximum value of the rectified input voltage

W

heat output

AT

Activation of the temperature control

RW

Determination of the reference value

VW

Determination of the comparison value and its deviation from the reference value

TR

Power control according to the temperature control

THERE

Query if temperature control is disabled

TE

End of temperature control

N

Signal for deactivating the temperature control is not available

Y

There is a signal to deactivate the temperature control

Claims (19)

1. Method of temperature regulation of a heating element (3) which is inductively heated by an inductor (6), to which an electrical power (P) is fed by way of a control circuit (2), characterised in that the temperature regulation is activated (AT) at a first time instant (t1) which is dependent on at least one electrical magnitude (v _o, i _o, P, I) of the control circuit (2), which depends on the temperature (T) of the heating element (3), a reference value (F_R) is determined (RW) at this first time instant (t1), that depending on the electrical magnitude (v_o, i_o, P, I) at at least one later time instant (t2 - t7) a comparison value (F_V ) and a deviation of this comparison value (F_V ) from the reference value (F_R ) are determined (VW) and that power (P) is fed to the inductor (6) in dependence on the deviation so that the temperature (T) of the heating element (3) is regulated (TR) to a constant value corresponding with the reference value (F_R ).
2. Method according to claim 1, characterised in that the temperature regulation is activated by a user by actuation of a control element (9).
3. Method according to one of claims 1 and 2, characterised in that the comparison value (F_v ) of the electrical magnitude (v_o, i_o, P, I) is determined at predetermined time intervals (t2 - t7).
4. Method according to claim 3, characterised in that the predetermined time intervals (t2 - t7) are periodic.
5. Method according to one of the preceding claims, characterised in that the electrical magnitude is the electric power (P) and/or a mean voltage and/or a mean current (I).
6. Method according to one of the preceding claims, characterised in that the reference value (F_R ) and/or the comparison value (F_V ) is or are an impedance of the heating element (3) and the inductor (6).
7. Method according to one of the preceding claims, characterised in that the reference value (F_R ) and/or the comparison value (F_V ) are calculated from the electrical magnitude (v_o, i_o, P, I).
8. Method according to one of claims 2 to 7, characterised in that the temperature regulation is deactivated by the user by actuation of the control element (9).
9. Method according to one of claims 2 to 7, characterised in that the temperature regulation is deactivated by the user through removal of the heating element (3).
10. Method according to one of the preceding claims, characterised in that the reference value (F_R ) and/or the comparison value (F_V ) is or are determined at a predetermined frequency of the electrical magnitude (ν _o , i_o ).
11. Control circuit for inductive heating of a heating element (3) by an inductor (6), with a power regulator (10) for regulation of an electrical power (P), which is fed to the inductor (6) and with a temperature regulation for the heating element (3),
    characterised in that the control circuit (2) comprises a control element (9) for activation of the temperature regulation, that the control circuit (2) comprises at least one measuring device (11) for determination of at least one element magnitude (v_o, i _o, P, I) of the control circuit (2), which depends on the temperature (T) of the heating element (3), that the control circuit (2) is constructed for determination of a reference value (F_R ), which is dependent on the electrical magnitude (v _o, i_o, P, I), at an activation time instant (t1) of the temperature regulation and for determination of a comparison value (F_V ), which is dependent on the electrical magnitude (ν _o, i_o, P, I), at at least one later time instant (t2 - t7), that the control circuit (2) comprises a comparison unit (12) for determination of a deviation of the comparison value (F_V ) from the reference value (F_R ) and that the control circuit (2) comprises a control unit (12) for controlling the power regulator, in dependence on the deviation, for temperature regulation of the heating element (3) to a constant value corresponding with the reference value (F_R ).
12. Control circuit according to claim 11, characterised in that the control element for activation of the temperature regulation is at least one switch (9) or at least one contact sensor (9).
13. Control circuit according to claim 11 or 12, characterised in that the measuring device (11) for determination of the electrical magnitude (v_o , i_o , P, I) of the control circuit (2) comprises a voltage measuring device and/or a current measuring device (18).
14. Control circuit according to claim 13, characterised in that the measuring device (11) comprises at least one current/voltage converter (18).
15. Control circuit according to one of claims 11 to 14, characterised in that the control circuit (2) comprises a microprocessor (12).
16. Induction cooking field with a control circuit (2) according to one of claims 11 to 15.
17. Induction oven with a control circuit (2) according to one of claims 11 to 15.
18. Induction oven according to claim 17, characterised in that the heating element (3) is a wall (23, 24) of the induction oven.
19. Induction oven according to claim 17 or 18, characterised in that the heating element (3) is a carrier for stock to be cooked.

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